We are working on a strategy for development of a live attenuated vaccine for AIDS that depends on deletion of genetic elements. Toward this goal, we have constructed virus strains with mutations in transcriptional control elements and analyzed their properties in rhesus monkeys. Rhesus monkeys (Macaca mulatta) were infected with five strains of simian immunodeficiency virus (SIV) derived from SIVmac239 containing deletions ( ) or substitutions (subst) in NF-kB and Sp1 binding sites. We have shown previously that mutations in these regions still allow efficient SIVmac replication in primary lymphoid cell cultures (Ilyinskii and Desrosiers, J. Virol. 70:3118-3126, 1996). Two animals were inoculated intravenously with each mutant strain of SIVmac239 NFkB, Sp1234, NFkB Sp1234, substSp12 and substSp1234. All but one of the infected animals showed an early spike in plasma antigenemia, maintained high virus burdens, had significant changes in lymphoid tissues and 6 have died with AIDS within the first sixty weeks of infection. One of the animals infected with the SIV strain NFkB Sp1234 showed lower levels of plasma antigenemia and lower virus burdens; the other animal infected with this same mutant strain died with AIDS 17 weeks after inoculation. No consistent novel mutations or reversions were detected in proviral sequences derived from the animals infected with the deletion mutants and the substSp12 mutant by 20 weeks post infection. Point mutated sequences were partially deleted in both animals infected with the substSp1234 strain. These results indicate that the NF-kB and Sp1 binding sites are not essential for the induction of AIDS by SIVmac239. They also provide indirect evidence for the importance of a novel enhancer element in the U3 region of the SIVmac LTR that is located immediately upstream of the NF-kB binding site within the C-terminal region of the nef coding sequence.